Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology that uses magnetic elements. For example, Spin Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM) uses electrons that become spin-polarized as the electrons pass through a thin film (spin filter). STT-MRAM is also known as Spin Transfer Torque RAM (STT-RAM), Spin Torque Transfer Magnetization Switching RAM (Spin-RAM), and Spin Momentum Transfer (SMT-RAM).
FIG. 1 illustrates a conventional STT-MRAM bit cell 100. The STT-MRAM bit cell 100 includes Magnetic Tunnel Junction (MTJ) storage element 105, a transistor 101, a bit line 102 and a word line 103. The MTJ storage element is formed, for example, from at least two ferromagnetic layers (a pinned layer and a free layer), each of which can hold a magnetic field or polarization, separated by a thin non-magnetic insulating layer (tunneling barrier). Electrons from the two ferromagnetic layers can penetrate through the tunneling barrier due to a tunneling effect under a bias voltage applied to the ferromagnetic layers. The magnetic polarization of the free layer can be reversed so that the polarity of the pinned layer and the free layer are either substantially aligned (parallel) or opposite (anti-parallel). The resistance of the electrical path through the MTJ will vary depending on the alignment of the polarizations of the pinned and free layers. This variance in resistance can be used to program and read the bit cell 100. The STT-MRAM bit cell 100 also includes a source line 104, a sense amplifier 108, read/write circuitry 106 and a bit line reference 107.
Those skilled in the art will appreciate the operation and construction of the memory cell 100. For example, the bit cell 100 may be programmed such that a binary value “1” is associated with an operational state wherein the polarity of the free layer is parallel to the polarity of the pinned layer. Correspondingly, a binary value “0” may be associated with an anti-parallel orientation between the two ferromagnetic layers. A binary value may thus be written to the bit cell by changing the polarization of the free layer.
FIG. 2 illustrates a conventional design of the MTJ cell 105 in more detail. An antiferromagnetic (AF) layer 204 is first formed on a bottom electrode 202, and then a pinned layer stack 220 is formed on top of the AF layer 204. The AF layer 204 may be formed from a platinum-manganese (PtMn) alloy, for example. The pinned layer stack 220 is “pinned” with a fixed magnetic polarization to form a pinned layer. The pinned layer stack may include one or more layers. The pinned layer stack 220 may sometimes be referred to as a composite AF pinned layer or a “synthetic” AF (SAF) pinned layer. The pinned layer stack 220 may include a bottom pinned layer 206 typically formed of a metal alloy such as cobalt-iron (CoFe) and/or cobalt-iron-boron (CoFeB), a coupling layer 208 typically formed of a non-magnetic metal such as ruthenium (Ru), and a top pinned layer 210 typically formed of a metal alloy such as CoFe and/or CoFeB. A tunneling barrier layer 212 is formed of an insulator such as a metal oxide like magnesium-oxide (MgO) on top of the pinned layer stack 220. A free layer 214 with variable magnetic polarization is formed on top of the barrier layer 212. In some designs, a capping or hardmask layer 216 such as tantalum (Ta) is formed on top of the free layer 214.
The conventional MTJ design of FIG. 2 has several drawbacks. One drawback is that high temperature back-end processes may allow diffusion of undesirable materials from one layer to another. For example, a high temperature annealing process for setting a desired magnetic moment of a PtMn AF layer 204 may mobilize Mn atoms and allow them to diffuse from the AF layer 204 into the bottom pinned layer 206, the coupling layer 208, the top pinned layer 210, and even as far as the tunneling barrier layer 212. Ru atoms of the coupling layer 208 may be similarly diffused throughout the various other layers by one or more high temperature back-end processes. The diffusion of undesirable elements throughout the MTJ stack may inhibit or destroy antiferromagnetic coupling of the pinned layer stack 220, decay exchange coupling between the AF layer 204 and the pinned layer stack 220, damage inter-layer interfaces (resulting in poor thermal stability, etc.), reduce the magnetoresistance of the MTJ cell, or even prevent the MTJ cell from being able switch between states. Accordingly, there is a need for an improved pinned layer stack for MTJ storage elements usable in STT-MRAM cells.